ipw2x00: silence GCC warning for unused variable 'dev'

Building the libipw component without CONFIG_LIBIPW_DEBUG set triggers this GCC
warning:
    drivers/net/wireless/ipw2x00/libipw_wx.c:526:21: warning: unused variable 'dev' [-Wunused-variable]

The cause of this warning is that, without CONFIG_LIBIPW_DEBUG set,
LIBIPW_DEBUG_WX compiles away. Fix it by substituting ieee->dev for (its
equivalent) dev.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>